International Journal of Trendy Research in Engineering and Technology
Volume 4 Issue 6 Oct’ 2020
ISSN NO 2582-0958
______________________________________________________________________________________________________________
Growth & characterization of Langasite crystals for SAW device
applications
N.Marimuthua, P.Suresh Kumarb*, R.B.Upadhyayc
a
.Department of Physics, Bharth University,Chennai , bFormer Professor, Velammal Engineering College, Chennai600 066, Tamil Nadu, India
c
Space application centre, Ahmedabad, Gujarat, India
*Corresponding author – suresrath@yahoo.com
Abstract
LGS crystal which melts congruently at 1470⁰C has been grown by Czochralski method. The langasite
crystal of length about 1cm grown along its Z-axis was cut in X and Y directions , polished and
subjected to various characterization studies. Phase and structure of the grown crystal was confirmed by
Powder XRD measurement. FTIR spectrum was carried out to confirm the functional groups present in
the grown crystals. The optical behavior was studied by UV–vis-NIR analysis. Electrical properties such
as Dielectric constant, Resistivity, Conductivity and Piezoelectric coefficient have also been studied.
Keywords:Langasite, Czocharlski technique, Piezoelectric ,SAW device,
Introduction
Piezoelectric materials play a very important
role in the modern industry. The most widely
spread products of piezoelectric engineering is
radio frequency (RF) control elements
(resonators) and selection (filters)[1, 2]. The
main material used in piezoelectric engineering
is quartz which is having highly stable
frequency-temperature characteristics and is the
reason for
its application in resonator
production. One of the piezoelectric materials
with high K2emcis lithium tantalate, enabling to
design wide band filters. However, due to the
low frequency-temperature stability and the low
quality factor (Q) of resonators made of this
material the application of these filters is
limited. Langasite(LGS) crystal belongs to
piezoelectric materials with the value of
electromechanical
coupling
coefficient
intermediate between quartz crystal and that of
lithium tantalate. LGS has attracted increasing
attention because of its excellent properties in
applications of surface acoustic wave, bulk
acoustic wave, and sensors fields[3,4]. An
important feature of LGS is that it undergoes no
phase transitions up to its melting point, which
stimulates the development of langasite- based
piezoelectric sensors[5,6]. In the present work
LGS crystal has been grown by Czochralski
method and subjected to XRD, FTIR, UV and
Electrical characterization.
II. Synthesis
The LGS polycrystalline material synthesis was
carried as per the following reaction
3 La2O3 + 5 Ga2O3 + 2 SiO2 ------- 2
La3Ga5SiO14
The synthesis of LGS has been carried out by
solid state sintering method from precursors of
99.99% pure La2O3, Ga2O3 and SiO2 weighed
according to stoichiometry ratio and then well
mixed in a planetary mill for 6 hours. To
compensate the loss of Ga by volatisation at
high temperature excess amount of Ga was
taken. Mixtures were then pressed at 10 kgf/cm2
to pellets and sintered for 12 h at 11000C.The
polycrystalline LGS pellet is shown in Figure 1.
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International Journal of Trendy Research in Engineering and Technology
Volume 4 Issue 6 Oct’ 2020
ISSN NO 2582-0958
______________________________________________________________________________________________________________
Fig.1.The prepared Polycrystalline LGS
pellet
Growth of Langasite crystal
Fig. 2. Photograph of grown LGS crystal
Langasite crystal was grown along its Z-axis by
the conventional Czochralski technique. Initially
polycrystalline LGS pellets were charged into
Pt/10%Rh crucibles of 50 mm diameter and 50
mm height. Pellets of LGS weighing 150 g
which is much less than the volume of the
crucible were taken in the crucible in order to
avoid overflow of material from the crucibles
when the pellets are molten. The crucible was
then placed on a crucible holder which can be
moved vertically along the furnace axis, thus
allowing an appropriate positioning of the
crucible within the temperature gradient of the
furnace. The basic process was to melt the
pellets completely and then to cool it to observe
the exact crystallization temperature of the
material for the present setup. Charge was
heated to 1600 0C , held at that temperature for
3 h for complete melting and then cooled down
slowly to find the solidification temperature.
After several runs the
crystallization
temperature was found to be 1480°C, and a
polycrystalline seed crystal (Figure 10) was
introduced at this temperature so as to initiate
the growth process by pulling and simultaneous
rotation of the seed rod. A pulling rate of 0.5-3
mm/h and rotation rate of 5-40 rpm were
maintained during growth process. The as
grown single crystal of size 38x22x9 mm is
shown in figure 2a.The X and Y cut crystal are
shown in fig.2b.
IV. Characterization
i) X-ray diffraction study
Powder X-ray diffraction analysis of
the
polycrystalline LGS prepared by solid state
reaction method and LGS crystal was carried
out using PANalytical-X-ray diffractometer in
the range 10 - 70° to confirm the structure and
phase. The Kα radiations from a copper target (λ
= 1.5406 Ǻ) was used.The Powder-XRD
spectrum of polycrystalline LGS and LGS
crystal are shown in Figure 3. From the PowderXRD spectrum, the formation of LGS phase and
structure was confirmed. The calculated cell
parameter values are a=8.1624 , c=5.093. The
diffraction pattern well agreed with the data of
standard for LGS (JCPDS No: 72-2249).
Fig.3. The Powder-XRD spectrum of
polycrystalline LGS crystal (a) and Grown LGS
crystal (b)
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International Journal of Trendy Research in Engineering and Technology
Volume 4 Issue 6 Oct’ 2020
ISSN NO 2582-0958
______________________________________________________________________________________________________________
UV-Vis-NIR absorption spectrum
The absorption spectrum of Langasite crystal
was recorded using Lambda 35 UV Winlab
Spectrometer. The UV spectrum was recorded
between 400 to 1100 nm. The recorded
absorption spectra of Langasite crystal is shown
in Figure 4. The absorption edge was observed
in the range 500 - 600 nm.
Fig.5 FTIR recorded spectrum of LGS crystal
Dielectric studies
Fig.4
UV
reflected spectrum of LGS crystal
FTIR Analysis
Figure 5 shows the From the FTIR spectra of the
sample shown in figure 5 the functional groups
are analyzed taking into account the molecular
structure of the material. The peak at 450 cm-1is
assigned to the O-La-O stretch. The absorption
bands in the region of 575 cm-1 are due to O-SiO stretching mode. Stretching at 628 cm-1and
735 cm-1are the evidence for the existence of
Ga-O group. The band at 675 cm-1signifies the
La-O stretching vibration. The Si-O stretching
vibration obtained at 779 and 879 cm-1.
Frequency dependence of dielectric constant for
Langasite crystal in the frequency range 100
Hz to 3 MHz at 313 Kis shown in figure 6. It is
observed that dielectric constant is high at low
frequency which is due to the contribution of
various polarization[7]. At high frequencies,
only electronic polarization with large relaxation
time exists and all other polarizations cease[8].
Hence the net dielectric constant decreases as
frequency increases. The dielectric constant for
the Langasite crystal at 1MHz was calculated
found to be 8.87.
Fig.6 Frequency dependence of dielectric
constant for LGS crystal at 313 K
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International Journal of Trendy Research in Engineering and Technology
Volume 4 Issue 6 Oct’ 2020
ISSN NO 2582-0958
______________________________________________________________________________________________________________
Tangent loss (tanδ) found to have higher value
at low frequency and decreases with increasing
frequency, illustrating the relaxation process.
The higher dielectric loss that occurs at lower
frequency may be due to an accumulation of
free charge. The polar ionization which occurs
due to the charge accumulation of decreases,
leads to a decrease in the value of the dielectric
loss.
Fig.8 Frequency dependence of AC conductivity
for LGS crystal at 313 K.
Fig.7 Frequency dependence of dielectric loss
for LGS crystal at 313 K
AC conductivity
Figure 8 shows the frequency dependence of
AC conductivity (σac) at 313 K for Langasite
crystal. The AC electrical conductivity (σac) of
the LGS crystal was calculated from the
following equation,
σac =ωɛrɛ0tanδ
where ω (=2πf) is the angular frequency, f is the
applied frequency. From fig.8, it is clear that the
conductivity is independent of frequency at low
frequency region whereas
above the
characteristic frequency the conductivity
increases with increase in frequency. The high
conductivity at higher frequencies confirms the
short-range intrawell hopping of charge carriers
between localized states[9].
Piezoelectric studies
The piezoelectric coefficient (d33) was measured
for the grown Langasite crystal using APC USA
make
YE2730A model
d33 meter. The
Piezoelectric coefficient of Langasite crystal was
found to be 5 pC/N.
Conclusion
Langasite single crystal of length 1cm was
grown by Czochralski technique . The structure
of LGS was confirmed by powder XRD. UVVis –NIR absorption spectrum shows absorption
edge at 525 nm. The various functional groups
present in the crystal were confirmed by FTIR
analysis. The dielectric measurements show that
the
dielectric constant and dielectric loss
decreases with increase in frequency. The AC
conductivity values found to have higher values
at high frequency. Piezoelectric coefficient of
the crystal was found to be 5 pC/N.
www.trendytechjournals.com
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International Journal of Trendy Research in Engineering and Technology
Volume 4 Issue 6 Oct’ 2020
ISSN NO 2582-0958
______________________________________________________________________________________________________________
IEEE Interl. Ultrason.Symp, 2, 16761679 (2009)
Acknowledgement
Author wish to acknowledge the financial
support from ISRO for carrying out this research
(ISRO/RES/3/664/2014-15)
[7]
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